1. Field of the Invention
This invention relates to a radiation image information read-out method and apparatus, wherein a technique for detecting light emitted from opposite surfaces of a stimulable phosphor sheet and thereby detecting two image signals from the opposite surfaces of the stimulable phosphor sheet is employed.
2. Description of the Related Art
In various fields, such as the medical field, techniques for reading out a radiation image of an object, which has been recorded on a stimulable phosphor sheet, to obtain an image signal, performing predetermined image processing on the image signal, and thereafter displaying a visible image, which is reproduced from the processed image signal, on an image display device, such as a cathode ray tube (CRT) display device, or outputting the visible image on film with a printer, such as a laser printer (LP), have heretofore been utilized. As apparatuses for reading out the radiation image, for example, computed radiography (CR) apparatuses, in which digital image processing techniques utilizing computers are employed, have heretofore been utilized.
The CR apparatuses are radiation image recording and read-out apparatuses. With the radiation image recording and read-out apparatuses, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of a stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal with photoelectric read-out means, such as a photomultiplier. Recently, the CR apparatuses are widely used in practice.
As techniques for photoelectrically detecting light emitted by a stimulable phosphor sheet, the applicant proposed techniques for detecting light emitted from opposite surfaces of a stimulable phosphor sheet and thereby detecting two image signals from the opposite surfaces of the stimulable phosphor sheet in, for example, U.S. Pat. No. 4,346,295 and Japanese Unexamined Patent Publication No. 8(1996)-116435. With the proposed techniques, two photoelectric read-out means are located respectively on opposite surface sides of the stimulable phosphor sheet. Also, stimulating rays are irradiated to the two surfaces or only to one surface of the stimulable phosphor sheet, and the light, which is emitted by the stimulable phosphor sheet when it is exposed to the stimulating rays, is photoelectrically detected on the opposite surface sides of the stimulable phosphor sheet.
Specifically, with the techniques for detecting light emitted from opposite surfaces of a stimulable phosphor sheet and thereby detecting two image signals from the opposite surfaces of the stimulable phosphor sheet, a stimulable phosphor sheet is formed by overlaying a stimulable phosphor layer on a surface of a transparent substrate, and a radiation image is stored on the stimulable phosphor sheet. The stimulable phosphor sheet, on which the radiation image has been stored, is fitted on a transparent holder, and two photoelectric read-out means are located respectively above and below the holder. In this state, the light, which is emitted from one surface side of the stimulable phosphor sheet when the stimulable phosphor sheet is exposed to the stimulating rays, is detected by the photoelectric read-out means located above the holder, and an image signal (a one surface side image signal) is thereby detected. Also, the light, which is emitted from the other surface side of the stimulable phosphor sheet when the stimulable phosphor sheet is exposed to the stimulating rays, is detected by the photoelectric read-out means located below the holder, and an image signal (an other surface side image signal) is thereby detected.
Thereafter, an addition operation is performed on the image signal components of the two image signals having been detected from the opposite surfaces of the stimulable phosphor sheet (i.e., the one surface side image signal and the other surface side image signal), which image signal components represent corresponding pixels on the one surface and the other surface of the stimulable phosphor sheet, and an addition image signal (a superposition image signal) is thereby obtained. With the addition operation, high frequency noise occurring at random in each of the one surface side image signal and the other surface side image signal can be smoothed. Also, since the emitted light is collected from the two surfaces of the stimulable phosphor sheet, the light collecting efficiency can be enhanced. As a result, a superposition image, which has good image quality with an enhanced signal-to-noise ratio, can be reproduced from the addition image signal.
Also, as techniques for obtaining radiation image information by utilizing stimulable phosphor sheets, or the like, subtraction processing (subtraction operation) techniques have heretofore been known. With the subtraction processing techniques, a plurality of radiation images are recorded under different conditions and are then read out to obtain a plurality of image signals. Thereafter, a subtraction operation is performed on the plurality of the image signals, and a subtraction image signal is thereby obtained. In this manner, a subtraction image corresponding to the difference between the plurality of the radiation images, i.e. a subtraction image in which only a pattern of a specific object part (hereinbelow referred to also as a pattern of a tissue, a structure, or the like) in the radiation images has been enhanced or extracted, is obtained. Basically, the subtraction processing techniques may be classified into a temporal (time difference) subtraction processing technique and an energy subtraction processing technique. The applicant proposed various energy subtraction processing techniques utilizing stimulable phosphor sheets in, for example, U.S. Pat. Nos. 4,855,598 and 4,896,037.
Also, in order for a signal-to-noise ratio to be enhanced over the entire frequency band of an image obtained from the addition operation or the subtraction processing, the applicant proposed a technique for performing masking processing on the image signal obtained from each of the opposite surfaces of a stimulable phosphor sheet, such that a parameter (a weight factor) appropriate for each of the opposite surfaces and each of frequency bands can be utilized, as disclosed in, for example, Japanese Unexamined Patent Publication No. 7(1995)-319092.
Stimulable phosphor sheets have various different specifications. For example, stimulable phosphor sheets may have different sensitivities and may be classified into stimulable phosphor sheets having a standard sensitivity and stimulable phosphor sheets having a high sensitivity. If the sensitivity of the stimulable phosphor sheet varies, a width of the image signal corresponding to a desired read-out range (which width is referred to as the read-out gain), and/or a center value of the image signal corresponding to the center point of a desired read-out range (which center value is referred to as the read-out sensitivity), or a frequency response of the obtained image signal will vary. Also, besides the sensitivity, the particle size of the stimulable phosphor, the stimulable phosphor layer constitution, and the like, may vary for different generations of the stimulable phosphor sheets and, as a result, the frequency response of the obtained image signal varies.
Therefore, in cases where the specifications of the stimulable phosphor sheet vary, if the read-out sensitivity and/or the read-out gain or the parameters for the operation processing, such as the addition operation or the subtraction operation, are kept at predetermined values, an appropriate image cannot always be obtained.
The primary object of the present invention is to provide a radiation image information read-out method, wherein a technique for detecting light emitted from opposite surfaces of a stimulable phosphor sheet and thereby detecting two image signals from the opposite surfaces of the stimulable phosphor sheet is employed, and wherein an appropriate image is capable of being obtained from appropriate read-out processing regardless of specifications of the stimulable phosphor sheet.
Another object of the present invention is to provide an apparatus for carrying out the radiation image information read-out method.
The present invention provides a first radiation image information read-out method, comprising the steps of:
i) exposing a stimulable phosphor sheet, on which radiation image information has been stored, to stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, and
ii) detecting light emitted from opposite surfaces of the stimulable phosphor sheet when the stimulable phosphor sheet is exposed to the stimulating rays, two original image signals representing the radiation image information being thereby acquired,
wherein a read-out sensitivity and/or a read-out gain with respect to at least either one surface side of the stimulable phosphor sheet at the time of the image signal acquisition is controlled in accordance with specifications of the stimulable phosphor sheet.
The first radiation image information read-out method in accordance with the present invention should preferably be modified such that the controlling is performed such that the read-out sensitivities with respect to the opposite surfaces of the stimulable phosphor sheet become approximately identical with each other, and/or such that the read-out gains with respect to the opposite surfaces of the stimulable phosphor sheet become approximately identical with each other.
In the first radiation image information read-out method in accordance with the present invention, as a technique for controlling the read-out sensitivity, one of various techniques may be employed, with which the level of the image signal corresponding to the X-ray dose of a desired read-out range, e.g. the center value of the image signal corresponding to the center point of the desired read-out range, can be altered. As the technique for altering the center value of the image signal corresponding to the center point of the desired read-out range, for example, a technique for altering the voltage (high voltage) applied to a photomultiplier, which photoelectrically detects the emitted light, or a technique proposed by the applicant in, for example, Japanese Unexamined Patent Publication No. 2(1990)-108175, wherein signal transform is performed in accordance with a read-out sensitivity (a S value) in exposure data recognizer (EDR) processing in which no preliminary readout operation is performed, may be employed.
Also, as a technique for controlling the read-out gain, one of various techniques may be employed, with which the width of the image signal corresponding to the desired read-out range can be altered. For example, it is possible to employ a technique, wherein a gain of an amplifier for amplifying the signal obtained from the photomultiplier is altered, or a technique, wherein signal transform is performed in accordance with a latitude (a L value) in the EDR processing in which no preliminary readout operation is performed.
Further, in the first radiation image information read-out method in accordance with the present invention, in order for the read-out sensitivity or the read-out gain to be set in accordance with the specifications of the stimulable phosphor sheet, a table, which represents correspondence relationship between the specifications oft stimulable phosphor sheets and the read-out sensitivities or the read-out gains for the stimulable phosphor sheets, may be prepared. Reference may be made to the table, and the read-out sensitivity or the read-out gain may be set in accordance with the specifications of the stimulable phosphor sheet.
The present invention also provides a second radiation image information read-out method, comprising the steps of:
i) exposing a stimulable phosphor sheet, on which radiation image information has been stored, to stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation,
ii) detecting light emitted from opposite surfaces of the stimulable phosphor sheet when the stimulable phosphor sheet is exposed to the stimulating rays, two original image signals representing the radiation image information being thereby obtained, and
iii) performing operation processing on the two original image signals to obtain an operation-processed image signal, which represents the radiation image information,
wherein parameters for the operation processing are set in accordance with specifications of the stimulable phosphor sheet.
The second radiation image information read-out method in accordance with the present invention should preferably be modified such that, by the utilization of the first radiation image information read-out method in accordance with the present invention, a read-out sensitivity and/or a read-out gain with respect to at least either one surface side of the stimulable phosphor sheet at the time of the image signal acquisition is controlled in accordance with the discriminated specifications of the stimulable phosphor sheet.
In the second radiation image information read-out method in accordance with the present invention, the operation processing performed on the two original image signals may be typically the addition operation or the subtraction operation described above. However,the operation processing is not limited to the addition operation and the subtraction operation.
In order for the parameters for the operation processing to be set in accordance with the specifications of the stimulable phosphor sheet, a table, which represents correspondence relationship between the specifications of stimulable phosphor sheets and the parameters for the operation processing, may be prepared. Also, reference may be made to the table, and the parameters corresponding to the discriminated specifications of the stimulable phosphor sheet may be set. At this time, in cases where the specifications of the stimulable phosphor sheet are such that the stimulable phosphor sheet allows detection of an image signal from its one surface alone, the parameters for the operation processing may be set such that an image is constituted from only the image signal detected from the one surface of the stimulable phosphor sheet.
In the first and second radiation image information read-out methods in accordance with the present invention, the specifications of the stimulable phosphor sheet should preferably be discriminated automatically. For such purposes, one of various techniques for automatic discrimination may be employed. For example, a bar code representing the specifications of the stimulable phosphor sheet may be attached to the stimulable phosphor sheet or a cassette, which accommodates the stimulable phosphor sheet. Also, the bar code may be read with a bar code reader.
The present invention further provides an apparatus for carrying out the first radiation image information read-out method in accordance with the present invention. Specifically, the present invention further provides a first radiation image information read-out apparatus, comprising:
i) stimulating ray irradiation means for irradiating stimulating rays to a stimulable phosphor sheet, on which radiation image information has been stored, the stimulating rays causing the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, and
ii) image signal acquiring means for detecting light emitted from opposite surfaces of the stimulable phosphor sheet when the stimulable phosphor sheet is exposed to the stimulating rays, and thereby acquiring two original image signals representing the radiation image information,
wherein the improvement comprises the provision of control means for controlling a read-out sensitivity and/or a read-out gain with respect to at least either one surface side of the stimulable phosphor sheet at the time of the image signal acquisition, the controlling being performed in accordance with specifications of the stimulable phosphor sheet.
The first radiation image information read-out apparatus in accordance with the present invention should preferably be modified such that the control means performs the controlling such that the read-out sensitivities with respect to the opposite surfaces of the stimulable phosphor sheet become approximately identical with each other, and/or such that the read-out gains with respect to the opposite surfaces of the stimulable phosphor sheet become approximately identical with each other.
The present invention still further provides an apparatus for carrying out the second radiation image information read-out method in accordance with the present invention. Specifically, the present invention still further provides a second radiation image information read-out apparatus, comprising:
i) stimulating ray irradiation means for irradiating stimulating rays to a stimulable phosphor sheet, on which radiation image information has been stored, the stimulating rays causing the stimulable phosphor sheet. to emit light in proportion to the amount of energy stored therein during its exposure to radiation,
ii) image signal acquiring means for detecting light emitted from opposite surfaces of the stimulable phosphor sheet when the stimulable phosphor sheet is exposed to the stimulating rays, and thereby acquiring two original image signals representing the radiation image information, and
iii) operation-processed image signal forming means for performing operation processing on the two original image signals to obtain an operation-processed image signal, which represents the radiation image information,
wherein the improvement comprises the provision of parameter setting means for setting parameters for the operation processing in accordance with specifications of the stimulable phosphor sheet.
As in the first radiation image information read-out apparatus in accordance with the present invention, the second radiation image information read-out apparatus in accordance with the present invention should preferably further comprise control means for controlling a read-out sensitivity and/or a read-out gain with respect to at least either one surface side of the stimulable phosphor sheet at the time of the image signal acquisition, the controlling being performed in accordance with the discriminated specifications of the stimulable phosphor sheet.
The first and second radiation image information read-out apparatuses in accordance with the present invention should preferably further comprise discrimination means for discriminating the specifications of the stimulable phosphor sheet.
With the first radiation image information read-out method and apparatus in accordance with the present invention, the read-out sensitivity and/or the read-out gain with respect to at least either one surface side of the stimulable phosphor sheet at the time of the image signal acquisition is controlled in accordance with specifications of the stimulable phosphor sheet. Therefore, in cases where a stimulable phosphor sheet having a different sensitivity is utilized, the image read-out operation can be performed by setting the read-out sensitivity and/or the read-out gain appropriate for the stimulable phosphor sheet, and an appropriate image can be obtained.
With the second radiation image information read-out method and apparatus in accordance with the present invention, the parameters for the operation processing, such as the addition operation or the subtraction operation, are set in accordance with the specifications of the stimulable phosphor sheet. Therefore, in cases where the particle size of the stimulable phosphor, the stimulable phosphor layer constitution, and the like, vary for different generations of the stimulable phosphor sheets, the operation processing can be performed with the parameters appropriate for the utilized stimulable phosphor sheet, and an appropriate image can be obtained.
Also, in cases where the specifications of the stimulable phosphor sheet are such that the stimulable phosphor sheet allows detection of an image signal from its one surface alone, the parameters for the operation processing may be set such that an image is constituted from only the image signal detected from the one surface of the stimulable phosphor sheet. In this manner, an image can be reproduced from only the image signal, which has been detected accurately from one surface side (e.g., the stimulating ray irradiation surface side) of the stimulable phosphor sheet.
Therefore, with the radiation image information read-out methods and apparatuses in accordance with the present invention, appropriate read-out processing can be performed regardless of the specifications of the stimulable phosphor sheet, and an appropriate image can thereby be obtained.